The small marks on four corners of the marked playing cards is in fact not so small. It's small when being compared with the special big marks but there are marks on the whole back of the marked deck. These marks are simply divided into 4 sets in four corners.
Our technicians mark the cards with clear luminous marks at the four corners by special machine. Even if you can see only one corner of the card, you can know what card it is by recognizing the number and suit at this corner, which is much different from the big marks or normal marks. And this is the very good advantage of the small marks. If you have good eye sight or see the marked cards in short distance, the small marks is ideal for you.
And sometimes when the marked cards of big marks are too clear to see and people may see any difference with naked eyes. We may mark that cards with small marks at 4 corners, which can't be visible to any naked eyes.